1. Field of the Invention
The present invention relates to a blanking die apparatus which blanks iron core pieces from a progressively fed sheet to form a laminated iron core and a method for manufacturing the laminated iron core by using the blanking die apparatus.
2. Description of the Related Art
For instance, a laminated iron core which forms a stator or a rotor of a motor is manufactured as follows. A belt-shaped sheet such as an electromagnetic steel plate is progressively fed to a blanking die apparatus, sequentially blanked or stamped out in working stations respectively arranged along a feeding direction of the sheet to form iron core pieces of desired forms and the obtained iron core pieces are rotated and stacked. Here, as shown in FIG. 5, a sheet 75 is moved in the blanking die apparatus (in an upper part of a lower die 76) in a state where the sheet 75 is lifted by using lifters 77 arranged in the lower die 76 of the blanking die apparatus. Guide parts 78 are respectively provided at both sides in a direction of width of the progressively fed sheet 75 to prevent a shift of the sheet 75 in the width direction.
In recent years, the width of the sheet used to manufacture the iron core pieces is increased so as to meet an enlargement of the laminated iron core with a large size of the motor and the thickness of the sheet is decreased. Accordingly, rigidity of the sheet is extremely lowered as compared with a sheet in the past. As a result, a problem (a failure in feeding) arises that when the sheet is progressively fed in the state where the sheet is lifted by the lifters in the blanking die apparatus, the sheet is bent downward, and the sheet is caught in, for instance, a blank die of the lower die during a feeding operation of the sheet. This problem frequently arises in a working station which blanks the sheet in an external form where the rigidity of the sheet is especially reduced. Thus, a countermeasure for the problem is required.
Thus, as shown in FIG. 6, in a lower die 80 of the blanking die apparatus, when a plurality of lifters 86 to 92 are provided respectively in peripheries of blanking holes 81 to 85 provided in the working stations arranged along the feeding direction of the sheet (not shown in the drawing), a lifting height of the lifters 92 arranged in the periphery of the blanking hole 85 is set to be larger than a lifting height of the lifters 86 nearest to an inlet side in order to lift up more an area of the sheet located above the blanking hole 85. Thus, even when the rigidity of the sheet is extremely reduced so that the sheet is liable to be bent, the sheet is prevented to coming into contact with the periphery of the blanking hole 85. See JP-A-2003-200296, for example.